A light scattering study of concentrated dispersions in nonaqueous media

Interparticle interactions between colloidal poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in non-aqueous media have been investigated using time-average light scattering. The problem of multiple scattering was avoided by using a binary mixture of solvents such that the colloidal particles were optically matched. This enabled the static structure factor to be measured and from the small scattering vector expansion the osmotic pressure to be determined. The softness of the pairwise interaction potential has been exposed using the Chandler-Weeks-Anderson perturbation theory. However, it is concluded that dispersions of the type studied can be reasonably well approximated by a hard sphere fluid model.     

A small-angle neutron scattering study of adsorbed polymer structure in concentrated colloidal dispersions

Poly(ethylene oxide) (PEO) adsorption on colloidal silica particles was studied by small-angle neutron scattering under the core-contrast-matching condition. The volume fraction profile of the adsorbed layer was derived by modeling the average layer scattering term. It was found that, with increasing colloid concentration, the adsorbed PEO layers collapse due to the repulsions between adsorbed layers on neighboring particles. At the same time, the correlation length in the adsorbed layer obtained by fitting the layer fluctuation scattering term was found to decrease, indicating that denser polymer layers are formed. These two observations are self-consistent.     

A small-angle X-ray scattering study of the interactions in concentrated silica colloidal dispersions

The structure factors of colloidal silica dispersions at rather high volume fractions (from 0.055 to 0.22) were measured by small-angle X-ray scattering and fitted with both the equivalent hard-sphere potential model (EHS) and the Hayter-Penfold/Yukawa potential model (HPY). Both of these models described the interactions in these dispersions successfully, and the results were in reasonable agreement. The strength and range of the interaction potentials decreased with increasing particle volume fractions, which suggests shrinkage of the electrical double layer arising from an increase in the counterion concentration in the bulk solution. However, the interactions at the average interparticle separation increased as the volume fraction increased. The interaction ranges (delta) determined by the two models were very similar. Structure factors were also used to determine the size and volume fraction of the particles. The values of the size obtained from the structure factors were slightly larger than those obtained from the form factors; this difference is ascribed to the nonspherical shape and polydispersity of the colloidal particles. The volume fractions measured by these two methods were very similar and are both in good agreement with the independently measured results.     

Small-angle neutron scattering study of concentrated colloidal dispersions: the electrostatic/steric composite interactions between colloidal particles

Small-angle neutron scattering was used to investigate the interactions in concentrated colloidal dispersions containing silica or polystyrene latex with adsorbed polyethyleneoxide (PEO). In these dispersions of charged particles, both electrostatic and steric repulsions are present. The PEO layer was made invisible to neutrons through contrast matching. The effect of the interparticle repulsion was clearly shown in the scattering spectra by the appearance of a peak at low Q. The effective potentials can be well described by the Hayter-Penfold/Yukawa (HPY) potential. In the silica dispersions studied, the layer thickness is small, hence the electrostatic potential dominates and the potential has a lower concentration dependence. In the dispersions of polystyrene latex, the adsorbed layer is thicker; consequently, the electrostatic potential dominates at low volume fraction (the potential has a lower concentration dependence), and the steric potential dominates at higher volume fraction (the potential has a higher concentration dependence). This study also suggests that when more than one potential is present the stronger one has a dominant influence in determining the structure factor. This finding makes it possible to describe the multicomponential interactions by a single function.     

Electrophoresis of concentrated colloidal dispersions in low-polar solvents

We present a method to accurately measure the electrophoretic mobility of spherical colloids at high volume fractions in real space using confocal laser scanning microscopy (CLSM) and particle tracking. We show that for polymethylmethacrylate (PMMA) particles in a low-polar, density- and refractive-index-matched mixture of cyclohexylbromide and cis-decahydronaphthalene, the electrophoretic mobility decreases nonlinearly with increasing volume fraction. From the electrophoretic mobilities, we calculate the ζ-potential and the particle charge with and without correcting for volume fraction effects. For both cases, we find a decreasing particle charge as a function of volume fraction. This is in accordance with the fact that the charges originate from chemical equilibria that represent so-called weak association and/or dissociation reactions. Finally, as our methodology also provides data on particle self-diffusion in the presence of an electric field, we also analyze the diffusion at different volume fractions and identify a nonlinear decreasing trend for increasing volume fraction.     

Hard-sphere perturbation theories applied to concentrated colloidal dispersions

The application of hard-sphere perturbation theory to monodisperse colloidal dispersions is examined in detail. Osmotic pressures and radial distribution functions are calculated by these theories and compared with exact Monte Carlo results to allow a critical assessment of the accuracy of the theories. For most conditions encountered in practical colloidal dispersions the predictions of zero- and firstorder perturbation theories are very accurate.     

Influence of polydispersity on the light scattering of concentrated dispersions of spherical particles

The light scattering of mixtures of hard spherical particles of two sizes is derived in the Percus-Yevick approximation, assuming that the light scattering intensity of each particle is proportional to its volume squared. A rather simple expression is obtained which is used to assess the influence of polydispersity on the light scattering of spherical particles at high concentrations.     

Small-angle neutron scattering study of concentrated colloidal dispersions: the interparticle interactions between sterically stabilized particles

Small-angle neutron scattering (SANS) was used to investigate the interparticle interactions in concentrated dispersions of colloidal silica stabilized either by steric or by electrostatic repulsive interactions. In 10 mM NaCl, an adsorbed PEO layer is required to prevent flocculation, and particles are stabilized by steric repulsions. The adsorbed layer was made invisible to neutrons by contrast matching with the aqueous continuous phase. Dispersions of the same particles at the same concentrations but in the absence of added salt and adsorbed PEO were also studied. In both cases, the SANS spectra of concentrated dispersions show a peak at low Q, which is due to interparticle interactions: a structure factor. The SANS data can be described rather well by a homogeneous spherical form factor and a structure factor based on the Hayter-Penfold/Yukawa potential model. The steric potential was compared to the electrostatic potential obtained by fitting the SANS data of the bare silica dispersions. The steric potential shows a greater dependence on the particle volume fraction, which we ascribe to the penetration and compression of the adsorbed PEO layer as the particles approach.     

Polyaniline dispersions 7. Dynamic light scattering study of particle formation

 When aniline is oxidized in the presence of colloidal silica, composite polyaniline–silica particles of submicrometer size are obtained. Dynamic light scattering was used to monitor the course of dispersion polymerization of aniline and the formation of particles. The increase of a hydrodynamic radius of particles was observed as polyaniline had been produced. Additional increase in particle size after polymerization has also been recorded. The rate of aniline polymerization was found to increase with increasing temperature in the range 0–50 °C. Well-defined particles are formed below 30 °C while above this temperature the colloidal stability of the resulting systems is limited. The activation energy of aniline polymerization was estimated. Received: 24 February 1997 Accepted: 7 May 1997     

Light scattering - diagnostic methods for colloidal dispersions

The increasing demand from the colloid research community for quick and noninvasive experimental techniques as well as the rapid progress of modern optics and electronics have led to a considerable expansion in the field of light scattering. This review introduces the basic concepts with some emphasis on novel approaches like the study of interacting particle systems, multiple scattering techniques, and non ergodic samples. Particular attention is then devoted to recent experimental progress towards more compact and rugged instruments.     

Frequency-dependent electrical conductivity of concentrated dispersions of spherical colloidal particles

This paper outlines the application of a self-consistent cell-model theory of electrokinetics to the problem of determining the electrical conductivity of a dense suspension of spherical colloidal particles. Numerical solutions of the standard electrokinetic equations, subject to self-consistent boundary conditions, are implemented in formulas for the electrical conductivity appropriate to the particle-averaged cell model of the suspension. Results of calculations as a function of frequency, zeta potential, volume fraction, and electrolyte composition, are presented and discussed.     

Stability of non-aqueous dispersions. Paper 7. Electrostatic stabilization of concentrated colloidal dispersions

The stability of colloidal dispersions in apolar media is reviewed beginning with dilute dispersions. A criterion for the definition between dilute and concentrated dispersions is presented followed by a new interpretation of Levine's theory for concentrated colloidal dispersions in apolar media [J. Colloid Interface Sci., 54 (1976) 34]. We conclude that concentrated dispersions containing submicron particles are generally unstable leading to coagulation. Successive coagulations lead to the formation of larger secondary particles, “coagulates”, which are ultimately stable to further coagulation and exist in energetic minima.     

Diffusion in concentrated dispersions: a study with fiber-optic quasi-elastic light scattering (FOQELS)

A fiber-optic, quasi-elastic light-scattering instrument is described using single-mode fiber optical components, including a novel slanted exit face optode. The setup operates with homodyne signal detection. It enables the characterization of diffusion processes in concentrated dispersions up to volume concentrations of 50%.The performance of the instrument is exemplified with results obtained from latex spheres with diameters of 226 nm and 404 nm at volume fractions from =0.01 to =0.5. The correlation functions are analyzed according to the second order cumulants method and the Contin-procedure yielding an average and a distribution function of the short-time self-diffusion coefficient,D _eff ^s , respectively.At high ionic strength the concentration dependence ofD _eff ^s /D ₀ is found to be in close agreement with theoretical predictions based on a multi-body interaction model of hard spheres up to =0.45. With decreasing ionic strength the negative slope of the virial expansion tends to increase, presumably due to enhanced repulsive electrostatic interactions.The described technology offers new experimental means for on-line remote control sensing of particle size in concentrated disperse systems.Presented at the 34. Hauptversammlung der Kolloidgesellschaft e.v., Bochum, Oct. 1–4, 1989     

Anticipating colloidal instabilities in cationic vesicle dispersions by measuring collective motions with dynamic light scattering

Vesicle dispersions are useful for many applications from medicinal to consumer products. However, using these dispersions requires some knowledge of and control over their colloidal properties. Measuring interparticle interactions between vesicles should allow framing the problem in terms of Smoluchowski kinetic models and consequently anticipating time-dependent aggregation and coalescence for the dispersions. However, this can be a difficult task for many complex mixtures. A primary goal of this paper is to show that it is possible to measure interparticle potential between small vesicles by measuring the concentration-dependent collective motion using dynamic light scattering. These measurements allow determination of the second virial coefficient for the dispersion, providing a convenient platform for summing all contributions to the interaction potential over all vesicle conformations, thus making the analysis of complex mixtures more tractable. As a verification of the approach, a comparison is made to dispersions in which the stability is governed solely by electrostatics, using existing techniques to anticipate instabilities. A second goal of this paper is to build a simple potential model in which the Smoluchowski model can be used to quantitatively anticipate the aggregation behavior of the small vesicle dispersion. Together, these observations constitute a convenient approach to anticipating the behavior of vesicle (and other) dispersions in complex mixtures.     

Dynamic light scattering by optically anisotropic colloidal particles in polyacrylamide gels

The translational and rotational motions of optically anisotropic spherical particles embedded in cross-linked polyacrylamide gels is studied by dynamic light scattering. The particles are liquid crystal droplets solidified in the nematic phase. The amount of cross linkers is varied to cross the sol-gel transition where the system becomes nonergodic for both translational and rotational diffusion modes of the probes. The translational and rotational dynamic correlation functions are obtained by measuring the intensity correlation function between crossed polarizers in the parallel and perpendicular geometries. Data from nonergodic systems is analyzed using an extension, to include rotations, of the method of Pusey and van Megen [Physica A 157, 705 (1989)]. Both diffusion modes are observed to be arrested as the rigidity of the gel increases.     

Ultra-small-angle X-ray and neutron scattering study of colloidal dispersions

The ultra-small-angle X-ray and neutron scattering techniques are useful techniques for the investigation of colloidal systems. The very high small-angle resolution of these scattering techniques has provided important and novel information to elucidate the formation mechanism of colloidal crystals. The Bonse–Hart optical system is expected to become a standard tool for investigating mesoscopic structures.     

Dynamic light scattering in turbid colloidal dispersions: a comparison between the modified flat-cell light-scattering instrument and 3D dynamic light-scattering instrument

It remains a challenge to measure dynamics in dense colloidal systems. Multiple scattering and low light-transmission rates often hinder measurements in such systems. One of the well-established techniques for overcoming the problem of multiple scattering is cross-correlation techniques such as 3D dynamic light scattering (3D-DLS). However, a high degree of multiple scattering, i.e., vanishing single-scattering contribution in the signal, limits the use of the 3D-DLS technique. We present another approach to measure turbid media by way of upgrading our flat-cell light-scattering instrument (FCLSI). This instrument was originally designed for static light-scattering (SLS) experiments and is similar to a Fraunhofer setup, which features a flat sample cell. The thickness of the flat sample cell can be varied from 13 mum to 5 mm. The small thickness increases the transmission, reduces multiple scattering to a negligible amount, and therefore enables the investigation of dense colloidal systems. We upgraded this instrument for DLS measurements by the installation of an optical single-mode fiber detector in the forward scattering regime. We present our instrumentation and subsequently test its limits using a concentration series of a turbid colloidal suspension. We compare the performances of our modified flat-cell light-scattering instrument with that of standard DLS and with that of 3D-DLS. We show that 3D-DLS and FCLSI only have a comparable performance if the length of the light path in the sample using the 3D-DLS is reduced to a minimum. Otherwise, the FCLSI has some advantage.     

Dynamic light scattering from spherical particles

The dynamic light scattering behavior of poly(butylmethacrylate) PBMA microgels and of kappa-casein micelles is compared with that from hard sphere latex particles. The latex particles and the kappa-casein micelles exhibited a single exponential decay of the time correlation function (TCF). For the microgels, progessively stronger deviations from a single exponential were observed as the scattering angle was made larger. These deviations are interpreted as being the result of internal modes of motion. From measurement of the first cumulant of the TCF, extrapolated towards zero angle, the translational diffusion coefficients D were determined, and the hydrodynamically effective radii were calculated via the Stokes-Einstein relationship. The ratio of the radius of gyration to the hydrodynamic radius was found to be?=0.775+0.012 for the latex particles, in good agreement with theory. The microgels, however, exhibit much lower?-parameters of 0.49 to 0.58, while the kappa-casein micelles showed the opposite behavior with values between 1.1 and 2.5. The results are interpreted on the basis of the DebyeBueche and Deutsch-Felderhof theory for porous spheres.